@incollection{AST_1974__19__107_0,
     author = {Helffer, Bernard and Zuily, Claude},
     title = {Non hypoellipticit\'e d'une classe d'op\'erateurs diff\'erentiels},
     booktitle = {Sur quelques \'equations aux d\'eriv\'ees partielles singuli\`eres},
     series = {Ast\'erisque},
     pages = {107--122},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {19},
     year = {1974},
     mrnumber = {364844},
     zbl = {0291.35015},
     language = {fr},
     url = {http://www.numdam.org/item/AST_1974__19__107_0/}
}